The purpose of the proposed research is to examine the role of vitamin B-12 in the regulation of folic acid metabolism and the effect of thyroid status on folic acid and vitamin B-12. The specific objectives include: 1. The study of the metabolic relationship between folic acid, vitamin B-12 and methionine with special reference to their effects on folic acid polyglutamate formation. 2. The study of the effects of thyroxine status on the function of vitamin B-12 and folic acid. 3. The study of the biological effects of the folic acid antagonist "x-methyl folic acid".